Water from a tap is heated in a pressure cooker, that allows a maximum gage pressure of 40 kPa, from 15ºC to boiling. The internal energy change is nearest:
A) 327 kJ/kg
B) 354 kJ/kg
C) 395 kJ/kg
D) 429 kJ/kg
C) 395 kJ/kg
(a pressure cooker prevents boiling until the pressure of 40 kPa gage, or 140 kPa absolute, is reached).
Find u1 at 15ºC (ignore the pressure). Then u2 = ug at 140 kPa. Since the mass is not specified, the specific internal energy is found:
u = u2 ? u1 = 458 ? 63 = 395 kJ/kg
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